Due to the improvements made in the performance of semiconductor integrated circuits, the circuit patterns which are drawn on an exposure photomask plate have become increasingly finer and higher in integration. In the production of such semiconductor devices, an inspection apparatus for detecting fine defects in the photomask plate and the circuit patterns which are formed in the plate has become necessary. For inspection of the cutting edge semiconductor exposure mask in which a DUV beam of wavelength near 193.4 nm from an exposure beam source constituted by an ArF excimer laser is used, inspection apparatuses which uniformly emit continuous or high speed repeating pulse output DUV beams, capture images of the patterns by CCD cameras etc., and process the obtained data are being used.
The wavelength of the DUV beam of an inspection apparatus is preferably as short as possible from the viewpoint of the improvement of resolution. On the other hand, shortening the inspection time in the production process is also important. For this reason, it is sought to raise the output of the inspection-use DUV radiation source and make it repeat at a super high speed so as to eliminate the need for synchronization with the CCD cameras etc., ideally to make it continuous. Further, as beam output, about 100 mW is necessary. For such a high output, continuous output DUV radiation source, the method of using a nonlinear optical crystal for wavelength conversion has been pursued as the only practical approach.
The fourth harmonic (wavelength 266 nm) of the fundamental wave emitted from a neodymium (Nd)-doped solid state laser or fiber laser (or fiber amplifier), the second harmonic (257 nm, 244 nm) of the fundamental wave emitted from an argon ion laser, etc. are typical outputs used, but in recent years, less than 200 nm DUV beams have been considered necessary. For example, the wavelength 198.5 nm continuous output DUV radiation source shown in PLT 1 is known. With this system, as described in NPLT 1, so far as the inventors know, this is currently the only continuous output DUV radiation source in which generation of a 100 mW or more beam by a wavelength of less than 200 nm has been reported. However, if a beam source enabling continuous output at near 193.4 nm—equal to the wavelength of an ArF excimer laser—can be practically produced as a DUV radiation source of a mask inspection apparatus, it would enable accurate evaluation of defects at the exposure wavelength. This would be far more useful than in the past and would greatly contribute to cutting edge semiconductor production.
To obtain a ultraviolet beam having a wavelength of 193.4 nm by wavelength conversion, it is possible to use one or more laser beam sources as the fundamental waves and combine these with generation of their harmonics or sum frequency mixing. In the past, various proposals have been made for systems for generation of 193.4 nm beams. However, the only systems by which actual generation of a 193.4 nm beam has been reported use pulse oscillation. So far as the inventors known, no continuous output beam source has been reported. There are various reasons, but this frankly means that no technically feasible system has been discovered which combines a laser beam source and nonlinear optical crystal to obtain a practical continuous output 193.4 nm beam.
In general, to generate a high output ultraviolet beam by wavelength conversion, it is required that (1) there be a nonlinear optical crystal which satisfies the phase matching conditions utilizing birefringence for the beam wavelengths for generation of harmonics and sum frequency mixing, (2) the conversion coefficient which is determined by the physical constants etc. of the crystal be high, (3) there be little absorption by the crystal at the generated wavelength, etc. As crystals which enable the generation of wavelength 193.4 nm beams, BBO (β-BaB2O4), LBO (LiB3O5), CLBO (CsLiB6O10), KBBF (KBe2BO3F2), etc. are known.
Among these, KBBF is currently the only crystal for which phase matching has been reported for the process of generation of a 193.4 nm beam by generation of the second harmonic of a wavelength 386.8 nm fundamental wave. However, it includes toxic substances and is difficult to grow as a crystal, so there is no prospect for practical use for consumer applications (NPLT 2). To obtain phase matching for generation of a 193.4 nm beam using other crystals, it is necessary to rely on sum frequency mixing of a long wavelength beam λ1 (λ1>386.8 nm) with a wavelength longer than 386.8 nm and a short wavelength beam λ2 (λ2<386.8 nm) with a wavelength shorter than 386.8 nm. The two wavelength beams λ1 and λ2 have to satisfy the following equation (1).1/λ1+1/λ2=1/193.4  (1)
Further, the phase matching conditions are expressed by the following equation (2) in the case of the usual co-linear type required for high efficiency wavelength conversion:n1/λ1+n2/λ2=n3/193.4  (2)
n1, n2, and n3 are the refractive indexes of the crystal at the wavelengths λ1, λ2, and 193.4 nm.
As systems which have been proposed in the past for combination of wavelengths in sum frequency mixing which satisfy the above conditions, there are the following examples.
(1) System by sum frequency mixing of λ1=2.075 μm and λ2=213 nm (FIG. 11: PLT 2),
(2) System by sum frequency mixing of λ1=1.55 μm and λ2=221 nm (FIG. 12: PLT 3, FIG. 13: PLT 4 and PLT 5, FIG. 14: PLT 6 and PLT 7),
(3) System by sum frequency mixing of λ1=1.415 μm and λ2=224 nm (FIG. 15: PLT 5),
(4) System by sum frequency mixing of λ1=1105 nm and λ2=234 nm (FIG. 16: PLT 6, FIG. 17: PLT 7),
(5) System by sum frequency mixing of λ1=1064 nm and λ2=236 nm (FIG. 18: PLT 8, FIG. 19: PLT 9 and PLT 10),
(6) System by sum frequency mixing of λ1=710 nm and λ2=266 nm (FIG. 20: PLT 11, PLT 12, PLT 13, PLT 14)
(7) System by sum frequency mixing of λ1=740 to 790 nm and λ2=256 to 262 nm (PLT 15).